Bisphenols obtained from the condensation reaction of ketones and phenols are valuable compounds useful in preparation of epoxy resins and polycarbonates. High quality epoxy resins and especially polycarbonates, which in optical applications must have high clarity, require especially pure bisphenols in their preparation.
There are many patents related to the purification of bisphenol; and the extent of the purification necessary is dependent on yield, crude bisphenol purity, and quality of final product desired. One method suggested is the formation of a 1:1 crystalline complex with phenol (U.S. Pat. No. 2,791,616). The phenol complex may be refined by washing with phenol, after which it is remelted and heated under vacuum to decompose the complex and distill out the phenol.
A number of suggested processes describe merely leaching crude bisphenol with a solvent or mixture of solvents selected to dissolve maximum amounts of by-products and minimum amounts of bisphenol. However, the bisphenol obtained from such solvent leaching normally is not pure enough for polycarbonate production. The bisphenol can also be purified by a combination of vacuum distillation and solvent leaching techniques (U.S. Pat. Nos. 3,219,549 and 3,290,391). More complicated, but more effective methods involve crystallization from an organic solvent at a temperature and pressure above the atmospheric boiling point of the solvent (U.S. Pat. No. 3,673,262). In yet another process for purification of bisphenols, a mixture of the reaction product, water and a water immiscible organic solvent is heated to a temperature below the boiling point of the organic solvent to provide two liquid phases which are then cooled to crystal bisphenol (U.S. Pat. No. 3,535,389). U.S. Pat. No. 3,326,986 employs a similar process in which the crude bisphenol is heated and melted in water without any organic solvent. The melt is agitated, then cooled and the crystals washed with a chlorinated organic solvent, e.g., methylene chloride, to remove the impurities.
Another more recent patent (U.S. Pat. No. 4,354,046) employs a crystallization in the presence of water and an organic solvent plus a treatment of the mother liquor with a cation exchange resin and recycling the product to the crystallization step.
Bisphenol produced from solvent crystallization always has some occluded solvent. Such occluded solvent can only be removed by remelting the crystals and destroying the lattice. Vacuum drying of the crystals at temperatures below their melting point fails to remove the occluded solvent. Vacuum distillation at elevated temperatures is a common method of purifying bisphenol A, but requires special equipment and care to exclude air to as not to cause color formation and product degradation. The present invention is directed to a method for removing the remaining small quantities of solvent to a parts-per-million level.